With tough questions being asked at City Hall over why San Francisco's parking meters aren't generating more revenue, one possible explanation beginning to emerge is the astonishingly high number of handicap placards that have been handed out in the city.

"While the MTA supports the legitimate use of disability placards, there's no doubt that they have an effect on our parking meter revenue," True said.

Tim Hornbeck is the executive director of the Arc of San Francisco, a nonprofit group that helps and advocates for people with disabilities. He agreed that better enforcement of handicap placards by the state DMV perhaps is needed. But so is parking meter enforcement, he said.

"I just walked outside of our building," Hornbeck said. "Out of 27 meters, nine were expired with no tickets, five had disability placards, and one was a city vehicle. Only 12 of those meters were getting revenue."

To obtain a handicap placard, California residents must have certain medical conditions -- such as heart disease, vision problems or impaired walking -- and have them certified by a doctor or other medical professional.

The uproar over a report revealing that the city's meters are capturing an average citywide of just 22 percent of their potential revenue showed no signs of abating Thursday. Supervisor Jake McGoldrick, who has waged an often lonely campaign for parking reform in the city, suddenly found himself in the spotlight, spending much of the day delivering on-camera interviews.

"Let's clean this act up," McGoldrick said, noting that doubling the meter collection could provide an extra $30 million for the San Francisco Municipal Railway.

Last year, the city collected about $29 million from meters that yielded on average $4.07 per day. City officials are at a loss to explain the relatively low average take, particularly from downtown meters that cost as much as $3 an hour.

In the city's downtown core, where meters are the most expensive, they pull in just $2.61 on average per day. In areas just outside downtown, in outlying commercial areas and at tourist-heavy Fisherman's Wharf, the average daily collections are $3.52, $4.41 and $5.59, respectively. Off-street meters in garages took in an average of $4.55 a day. There are about 373,115 registered cars in the city.

Those numbers are raising eyebrows because they mean that in a city where competition for street parking is sharp, meters downtown collected less than one hour's worth of revenue a day.

Just outside downtown, in outlying commercial areas, in off-street garages, and at Fisherman's Wharf meters were in use between 1 1/2 hours and nearly three hours per day, based on meter revenue from those zones.

Municipal Transportation Agency officials point out that thanks to better technology and a rise in parking rates, revenue from parking meters rose from $12 million in 2002 to the $29 million in 2006. That change is attributed to replacing mechanical meters, which frequently broke and were easily tampered with, and increases in parking meter rates.

Agency officials say they don't believe its employees -- or anyone else -- are stealing meter revenue as was the case in the 1990s, saying new collection and banking procedures make large-scale theft unlikely.

Aside from the high number of disabled placards, agency officials blame the low collection rate on such factors as commercial loading zones and parking reserved for construction. The city's metered spaces for motorcycles also are in lower demand.

Donald Shoup, a UCLA professor who has studied the issue, said sharp increases in parking meter rates such as those that have been imposed in San Francisco can have the effect of driving motorists into private garages.

Which is just what many Chronicle readers said they do now for their convenience and to avoid a costly parking ticket at an expired meter.

"If metered parking is the same cost hourly wise as a parking garage, I'm going to choose the parking garage," wrote Henry Yip, one of dozens who emailed The Chronicle their views on the issue. "I won't get a $40 ticket if the meter expires on me."

Shoup called San Francisco's low collection rate "just amazing," adding: "I think it just surprises everybody who sees the data."

He said some U.S. cities return their parking meter revenue to individual neighborhoods and business districts, giving local neighbors and residents a stake in enforcement.

"When it goes to the city, nobody is eager to say we ought to enforce payment of the meters," Shoup said. "If neighborhoods knew what the meters were producing, they would look very carefully at the books."

Shoup said handicap placards -- and their abuse -- is a problem facing many cities.

He said he believes one solution is ending meter time limits and eliminating free parking for people with disabilities, who often need to park for long periods and don't have the mobility to feed a meter.

"All you have to do is look out your window and see how many spots are occupied by cars with disabled placards," he said. "If you ended time limits, parking becomes like milk or gasoline or any other commodity. You pay for what you use."